Physics extra question thread

[dbhvt]

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...the acceleration is constant, meaning no change in velocity.

Repeat this mantra to yourself in the bathroom mirror for 10 minutes every morning untill Saturday:

"Acceleration IS a change in velocity. If there is acceleration, the velocity is changing."

If the acceleration is constant, and it is not zero, there IS a change in velocity. Always, every single time, no matter what. It could be a change in the direction, the speed, or both. On the MCAT, acceleration problems will generally be constant non-zero acceleration. All of those problems will involve a change in velocity. Everyone, everytime, every year, every administration. Now repeat after me:

"Acceleration IS a change in velocity. If there is acceleration, the velocity is changing."

HINT:

Try to stop thinking about speed. Speed doesn't really make sense. If you're going 50 mph, do you care what direction you are going? Yes, of course you do. Doing your thing around town do you ever care about speed without direction? No.

Speed is an abstract silly thing that is used when bragging about the Porsche they give you if you get a 45 on the MCAT. In physics and real life, you always deal with direction at the same time, so you want to think about velocity.

...Thanks so much for your help! One more quick question...If you throw an object up in the air the time it takes for it to reach the top is longer than the time it takes during its descent right? Is it because on the way down it has the force of gravity pulling it down so its velocity is greater than the way going up? If not, can you tell me a scenario when this would be true?

FAST AND EASY ANSWER:
It takes the same amount of time to go up as come down.


EXPLANATION:


Any object near the surface of the earth (ie, not the moon or a spaceship--closer than 300km for the quantitative folks), say a ball, or a block, or a person, will have the same force of gravity acting on it all the time everywhere.

If you throw a ball in the air and are worried about the time when the ball is in the air, what forces are acting on it?

Things your touching: nothing
Gravity if you need it: check
Friction if you need it: lets say you don't need air resistance (the ball is small and moving slowly)

So, you have a constant force acting on a particle. That force is a vector pointing down with a magnitude of (mass of the ball)*10m/s^2. That force is constant.

The force is constant, the acceleration is constant, the velocity is changing at a constant rate for as long as the ball can be described by the force diagram we just talked through (as long as nothing else is touching it--like the ground, for instance).

So describe the motion of the particle starting with upward velocity, but under constant downward acceleration:

The ball's velocity will change by 10m/s down every second of it's flight. Constant force, constant acceleration, constant rate of change in it's velocity. As far as the speed is concerned, the ball will keep moving up for some amount of time, but slow down at a constant rate untill it stops. Then it will start moving down. Because the rate it slows down on the way up is equal to the rate it speeds up on the way down, it takes the same amount of time to go up as it does to come down, and at any given height, it will have the same speed on it's way down as it did earlier on it's way up. In the up path, at 5 ft, the balls velocity might be 15 m/s up. In the down path, at 5 ft, the balls velocity would then be 15 m/s down.